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What is the significance of the 10,600 lbs MTOW limit?
I recently noticed that several airplanes have MTOWs very near or exactly 10,600 lbs. That was an interesting phenomenon to me because I am familiar with the 12,500 lbs MTOW regulatory limit on certain types of small planes, and familiar with the 19,000 lbs MTOW regulatory limit on certain other "commuter" planes, but I couldn't find anything to support why so many airplanes would have MTOWs at 10,600 lbs.
As I tried to google for "10,600 lbs max takeoff limit" I found more planes that are at or very near that exact MTOW but couldn't find the basis for why this is.
Does it have to do with European regulations? Does it have to do with insurance considerations?
Airplanes with near or exactly 10,600 lbs MTOW

Cessna CitationJet / M2
Beechcraft King Air C90 / and derivatives
Embraer Phenom 100
HondaJet HA-420

EDIT: A Little More Info
I have noticed that 10,600 lbs is very close to 4,800 kg. So its possible that this regulation stems from something non-FAA, something that regulates in kg, but even still, I haven't had any more luck with Google trying to figure out what the derivation of a 4,800 kg limit might be. Yet again, all I found was yet more planes that have that same MTOW limit.

It may have something to do with taxes, depreciation schedules and other ways to reduce taxes on the aircraft.  I know some EASA countries have different taxes for aircraft above and below 17,000 MTOW.

I can't find the specific regulations now, but I believe there are differences in requirements and permitted operations for "Very Light Jets", which are set at/near that 10,600 lbs level. Very Light Jet (VLJ) is a comparable designation to the "Light Sport Aircraft" (LSA), although obviously at a different point on the scale

Ive asked a friend who flies Citations if there is any significance to that figure. I'm not entirely sure its not just a coincidence

@JonStory, from doing a little googling on your idea, it does seem like part of the conventional industry definition for a VLJ is "up to 10,000 lbs MTOW" however I didn't easily find any FAA regulations that would contribute to that limit nor any explanation as to why all of these planes would be so consistently over that limit by ~600 lbs.

Remember it may not be an FAA (US) restriction, it could be one of many other CAA's - or it may be some kind of rating for certain runway types. On the other hand it could just be a convention based on the size/capacity/performance characteristics of these jets

@JonStory I considered that as well although it seems awfully coincidental. Its interesting to note that all 4 of the examples I provided have completely different engines, so not a constraint based on a particular engine.

I'd be surprised if it was not a number originally in kg, and I run into that a lot while researching regulations...


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Why are Search & Rescue aircraft looking for the OceanGate Titan submarine not visible on ADS-B Exchange?
Currently, there is an ongoing Search and Rescue mission being conducted by Canada, the United States, and commercial operators to find and rescue the crew of the OceanGate Titan submersible vessel (submarine) in the area of the Titanic.
Reportedly, this Search and Rescue mission involves multiple aircraft from Canada and the United States, including multiple C-130 aircraft as well as at least one aircraft providing sonar equipment.
Why are these aircraft not visible on sites like ADS-B Exchange?
For reference, this is a snapshot of the air traffic reported on ADS-B Exchange at 10:35 UTC on 21 June 2023 (daylight for over an hour in the search area):

ADS-B is line-of-sight.  For an aircraft to be visible to an ADS-B receiver, it needs to be above the horizon.  The wreck of the Titanic is 600 kilometers from the nearest land, so in order to be visible, an aircraft would need to be flying at an altitude of 28 kilometers/90,000 feet.  Going the other direction, an aircraft doing a search pattern at 3000 meters/10,000 feet would only be visible out to 200 kilometers/120 miles.
As DeltaLima notes, many military aircraft don't transmit ADS-B messages, but I don't think that's the case here.  I live near a flight path for USAF aircraft, and my ADS-B receiver regularly spots "cargo-like" military aircraft such as the P-8 Poseidon and C-130 Hercules being used here.  It's only the combat aircraft that usually fly with ADS-B off.

You are absolutely right. I was under the impression that ADS-B exchange had some feed from a few satellite based ADS-B receivers, but it turns out all the intermittent oceanic reports are over ADS-C. ADS-B can see a bit beyond the horizon though. At 1090 MHz the signal is following the curvature of the earth a little bit. Under good conditions I have had messages from 700 km away (aircraft at cruise altitude of 38000ft, receiver antenna at ~15 m AMSL)

I was under the impression that most military aircraft do transmit ADS-B when they don't need to be worried about being tracked, this includes fighter jets and other combat aircraft. I would assume this is for safety so civilian aircraft can stay out of their way.

Since the only aircraft that regularly approaches 90.000 feet is the SR-71, it's handy that it came equipped with all sorts of high resolution cameras. Unfortunately, flying examples are fairly few and far between...

@DeltaLima, you can get the occasional over-the-horizon message (that's probably how G-XLEK is visible in the screenshot in the question), but nothing reliable enough to track the search aircraft.

@Crazymoomin I see Blackhawks regularly flying by which show up on commercial ADS-B sites, but have never seen a fighter jet... of course, if there's already airspace restrictions in place they probably don't have to worry about civilian aircraft staying out of their way as much.

"ADS-B is line-of-sight": Yes, but aircraft crossing the ocean are visible on ADS-B Exchange. Their position is not reported via ADS-B, but via CPDLC, and it seems ADS-B Exchange has a feed for ADS-C which carries CPDLC messages, possibly by HF, satellite or VHF, depending on the conditions. So line of sight is not the reason some aircraft cannot be tracked.

@Michael I suspect they'll have it on from base to TRA and vice versa, but might turn it off within the TRA.

The aircraft probably do not transmit ADS-B messages, many military aircraft don't. The only way ADS-B exchange can locate these aircraft in such case is by using multilateration.
Multilateration only works if the aircraft is transmitting messages on 1090MHz AND there are multiple receivers receiving those messages. ADS-B exchange seems to only have intermittent signals from satellites over the ocean.

Since there is no abundance of receivers in the ocean, multilateration cannot work. Hence nothing is shown.
Please see this answer from Mark as well, it points out that the  aircraft are out of the range of terrestrial ADS-B receivers. Since ADS-B-Exchange does not have a feed from satellite based ADS-B receivers, their website does not cover the aircraft over the oceans unless the aircraft report over ADS-C.

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Why aren't test planes remote controlled?
There was a recent accident where SpaceShip2 broke apart in mid-flight, killing one and injuring another.
Given that we can remotely control drones from the other side of the world, why couldn't we do the same with this test airplane?
Considering that it's all launched locally, it would not even have the lag that drones have.

Because sometimes advancing science and society is more important than the safety of a single person.  Test pilots know that - and accept the risk.

Today's highly automated airplanes should be not too hard to control remotely - after all, all the cockpit control inputs become electrical signals, and at that point a multichannel remote could be plugged in. The Russians even flew their Buran version of the Space Shuttle autonomously, using a neural network to command the craft on its single flight in 1988.
However, a test pilot flies an aircraft with all his senses. Without the acceleration, a full 3D-view and the force feedback from the controls, he doesn't get the full picture when sitting in a control room and looking at screens full of data. That is done by test engineers anyway, and the pilot's job is to complete the observation of a new airplane's behavior by exposing himself to it.
You might argue that the 3D-view is just a matter of several cameras and displays, but the needed telemetry bandwidth for this would be immense. In a normal test flight, already hundreds of parameters are measured many times a second and transmitted to a ground station, where specialists for each system watch for anomalies in the data. It is much better to spend the telemetry budget for this than to send pictures down. The lack of acceleration and vibration alone would deprive the test pilot of an essential part of the picture, because it simply cannot be realistically recreated in a ground station.
Also, the inevitable delays incurred by radio transmission and processing for display will quickly put a pilot out of a loop which he could easily control if he were sitting in the airplane. The range of controllable eigenfrequencies would be reduced to phenomena happening at maybe 0.2 Hertz or less, whereas a pilot in the cockpit can comfortably stay ahead of oscillations with 0.5 Hertz or less.
However, it must be said that sometimes the test pilot does turn out to be a burden for the progress of the flight. On the second flight of the British TSR-2 prototype, one fuel pump began to develop an imbalance, oscillating just at the eigenfrequency of the pilot's eyeballs. He had to throttle back one of the engines to regain his vision.

Taking a plane that is designed to be operated by humans and retrofitting a system for remote control would be a big task. They do it with old fighter jets for use as target drones. But a target drone's task is pretty simple; fly like a fighter jet until it gets shot down.
At the point where SpaceShipTwo is, a bunch of testing has already been done. This is supposed to show that the vehicle is safe enough to move on to piloted testing. But testing aircraft and spacecraft is a dangerous job and things happen.
Creating a remote control system for an spacecraft like SpaceShipTwo would be much more of an issue. First of all, there will most likely be physical changes required, whether this is to install hardware to manipulate controls, or just electronics to remotely operate the flight computers. These physical changes will be extra work and will affect the tests, since when the plane is actually operating, those things won't be in place. And if those remote control systems fail, you lose the aircraft. Sure, you don't lose pilots, but why lose a plane just because a remote control system (which won't be used on production) fails? Should you spend time and money developing a complicated remote control system, or developing the actual vehicle?
Also, many tests require the pilots to be on board. They are not just testing the systems, but how the pilots interface with them. Having real pilots actually flying the vehicle is the whole point of this phase of testing.
See this related question as well: Why do the engineers need to be on board during testing?

also ping, it plagues RC guys just as much as gamers

and how do you test whether the control inputs work if you bypass them through some RC system? hmm...

Also note that orbital vehicles loose radio connection during reentry as air heated up to plasma temperature is conductive and shields radio waves. While SpaceShipTwo is a lot slower than orbital vehicles, it might be fast enough for this to be an issue.

Remote control systems have bugs and problems the same as any other system.
If you install a remote control system, you now have two systems that need to be tested, checked, maintained and working properly.
If there's a problem, and the vehicle crashes, you're immediately left with two possibilities: maybe the aircraft had a problem, or maybe the remote control system had a problem.
If the remote control system had a problem, maybe your airplane was fine, and you just lost a perfectly good plane due to a bug in your test system!

Counterpoint: if the remote-controlled airplane crashes, maybe the airplane had the problem, and you just avoided the need to hire a new test pilot.

Barnes Wallis asked this question in the 1940s.
The loss of life in the Dams Raid deeply affected him and he was adamant that he would not risk human life on his more experimental post-war designs until their basic concepts were proven through unmanned flight. (Source for this and details below)
These included the revolutionary Wild Goose and later Swallow designs, where variable geometry wings were used, not just to improve wing efficiency across a range of flight conditions (as in the much later F-111 and Tornado) but to eliminate the tail and its associated weight and drag penalties altogether. Wild Goose flew (remotely controlled) in January 1950.
However testing seems to have been slow and remote control techniques of the time didn't help. Without onboard cameras, at least one model was crashed by a pilot misjudging its position from a distance.
Swallow (in the form of a rocket propelled model) is reported to have flown stably at Mach 2.5 in 1955.
Neither progressed to manned prototypes, thanks in part to financial difficulties in post-war Britain.
In any case the intent was to prove the basic concepts rather than perform all testing in unmanned form, experimental manned planes were to be built along proven aerodynamic lines.

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What is the specific fuel consumption and cruise thrust of PW1521G engines?
I'm looking for the specific fuel consumption (SFC) and thrust related to PW1521G in cruise regime, as well as the associated assumptions for these ratings. I can't find them online.

"The" cruise thrust is whatever you need for "this" flight, at your planned cruise speed & altitude & gross weight. Change any of those parameters & the cruise thrust will change accordingly. This question looks to be unanswerably vague to me. VTC.

Gosh, I'd like to think that the answer to the OP ought to be found at the P&W website...

EASA type certificate for the engine. Note that SFC and thrust in "cruise conditions" is something commonly found in literature since pretty all flights will be conducted in optimum (economically speaking) conditions for cruise, eg here is a comparison of RR engines SFC/trust/altitude... for cruise, source. To me it's a valid question.

These are data that are commonly found in the literature, but I have failed to find them for this Pratt & Whitney engine. This is why I'm picking the brain of this forum. I'll put a bounty on this question once it's available.

"I have failed to find them", me too, but this is a valid question, that was my point. It would be better if you could adjust your question, the stack is ran in strict mode and good questions are often closed because they are not asked with all explicit details. E.g. here the manufacturer has designed the engine using some cruise conditions that need to be provided with the answer.

Have voted to re-open this question. It is a valid question, related to Aviation. All pilots understand what cruise conditions are.


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Where to find fuel type & fuel consumption info for PW 1000G engines family?
Where I can find the information about fuel type and fuel consumption for PW 1000G engines family?

It's a commercial jet engine, so it runs Jet-A. Fuel consumption depends on power setting, airspeed, temperature, and pressure. It would probably be more effective to ask about typical fuel consumption for a specific aircraft equipped with PW 1000G family engines.

It may be hard to find fuel consumption, especially since no planes with these engines are in service yet.

To raptortech97  - Jet A, or may be Jet A-1?

This database has fuel flow information as well as emissions data, but I did not see the PW1000 series (or LEAP) in there yet. It will be operated around the world, so it will be able to use Jet A or A-1.

In this presentation from 2009 PW quote savings of 140 gallons on a 500 nm trip. I presume they are comparing it to current engine options such as the IAE V2500.

I see only "savings", not absolute consumption value.

@Y.N I have noticed that engine makers tend not post absolute values until they enter operation. Possibly because it is partly dependent on the type of aircraft it's fitted on.

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How to have a conversation about safety with new GA pilot?
I have a family member who has been an amateur VFR pilot for a year.  I know very little about aviation but am concerned about his plans to pick up his new aircraft in Eastern Canada and fly it back home across Canada in February.
How can I talk to him about this in an intelligent and engaging manner that will have him properly evaluate the risks?
It seems that it would be potentially very dangerous and something that I would never consider doing but he is a young cocky fellow and I am worried.

Update:
My concern is that he is tackling too many variables at once. He has no familiarity with the plane as he is purchasing it. He has never flown cross country. The journey is from New Brunswick to British Columbia. The prairies have a lot of fog in the wintertime. The plane is not rated for icing. He will be crossing the Rocky Mountains. Airports are not necessarily closely spaced in case he needs an alternative landing due to weather or mechanical issues. My exposure to aviation was hang gliding where it was drummed into me to only change a single variable at a time.

I would never consider doing do not overestimate the survior bias https://imgs.xkcd.com/comics/survivorship_bias.png you think you made wise choices and you are now alive because of that, but that night in 1985 when you drove on icy road with poor tires was much more dangerous than your family member plan ...

I would much rather fly over Canada in Feb than drive across it, as my son & DIL did last year...

Just for completeness, where exactly is he flying from and to? There's a big difference between Montreal->Winnipeg and St. Johns->Vancouver - such as a factor of 3 in the distance.

It might help if you look at crash reports together. Most of them shows that the crash started even before takeoff. I might suggest seeing this youtube film: https://www.youtube.com/watch?v=ViO1j1iYn18  -- it is about the fatal crash of the  Youtuber TNFlygirl.

@FreeMan Although it would be depend on the weather, the aircraft, and the vehicle, I am inclined to agree with you.  But there is another option: to do neither and wait for a better time for a new pilot to fly their new aircraft.

@electron_positron  -  What exactly are your concerns?

@Dave Gremlin My concern is that he is tackling too many variables at once. He has no familiarity with the plane as he is purchasing it.  He has never flown cross country.  The journey is from New Brunswick to British Columbia.  The prairies have a lot of fog in the wintertime.  The plane is not rated for icing.  He will be crossing the Rocky Mountains.  Airports are not necessarily closely spaced in case he needs an alternative landing due to weather or mechanical issues.  My exposure to aviation was hang gliding where it was drummed into me to only change a single variable at a time.

@electronpositron I copied your concerns into your question for you, as they improve the quality of your question.

If he needs an emergency landing just in the middle of nowhere between two very distant airports, what is his plan to survive some days until being rescued? Clothing, food, sleeping bag, tent, lamps, cooker, satellite phone?

You stated you "...know very little about aviation...". Based on this I'm going to have to ask whether you have considered your ability to assess the safety of airborne operations?
Pilot training, even in the "basic" PPL level is very much safety oriented. Each pilot completing the training will have the means to assess risks involved in different kind of operations. This will not, of course, fully mitigate the inherent danger of operating aloft. Every now and then a Darwin award will be granted to a member of the pilot community, on top of the unfortunately inevitable statistical losses caused by factors that are not humanly manageable.
So in the end I guess, your concern is a viable one. What I would do, to avoid unnecessary conflict with the cocky young "eagle", is to ask him to explain to you how such a mission is planned and executed. Not in a way that can be interpreted as challenging his abilities, but as a question of genuine interest into the matter.
You should get a detailed plan describing schedule, route, alternate routes, emergency fields and diversion tactics. How and when to make the go/no go decision, when to divert. What to do when something breaks down. You might enquire specifically how does one manage a sputtering engine, and an engine that quits altogether. What will be in his emergency package. Communications with ATC...
All of that should be laid out to you in a coherent manner, especially the emergency routines should come without hesitation, accompanied with pointing and hand movements simulating the actions.
Any hiccups with those, and the flight plan to be existing only in electronic form (GPS, iPad, etc.) would be concerning. As John K suggested, a very fair question would be "Have you discussed this with an instructor / CFI / other seasoned pilot?"
There's absolutely no shame in confessing you are afraid. Saying that out loud would act as a perfect reason to ask for the aforementioned question.

and the flight plan existing only in electronic form => why is that?

@JonathanReez always carry a paper version of your flight plan and route drawn on the map. Electronics are far more likely to fail than paper :)

@Jpe61   Thanks, the topics of conversation you bring up are helpful.  I am genuinely interested in his trip and these topics will give me a better starting point to have a conversation than me just saying I am worried.

@Jpe61 would the flight plan not yet being on paper still be concerning considering that the flight will be in about three months?

Good point @Someone I'll edit for clarity.

The appropriate thing for a loved one, who is an "outsider" to aviation, to do is ask him to sit down with someone experienced in doing long ferry flights in single engine GA airplanes to go over his trip plan.
Nothing wrong necessarily with doing it in winter, especially Feb which tends to be better weather, if colder.
No thunderstorms for example. But for a long ferry trip the main thing, besides having proper winter gear and knowledge of cold weather operations, is having a schedule that allows for significant down time and a budget to finance that down time.
If he has a job to return to, and allowed only a day or two for delays, it's the "get home itis" that will probably get him in trouble as he pushes himself into making bad decisions when it's wiser to wait out weather. For any kind of long range cross country flying, schedule flexibility that minimizes time pressure is probably the #1 safety factor outside of competent basic airmanship.
Maybe ask around to find a pilot that your family member knows, with that sort of experience, and get them to bring it up to them with a "I hear you're planning a big ferry trip" sort of introduction and get into it that way. They may end up talking him into delaying the trip until June, generally the month with the least bad weather.
Not much else you can do beyond that.

Just tell them that:

new things = inconvenient unexpected (minor) technical problem

It is true for cars, ships, even for bicycles: the bottom bracket is a rather simple mechanical part, it either breaks after 50 km or after 20'000 km.
For a new, recently assembled plane it can be something very simple, but very costly in time. Tell them that they may have a simple issue forcing them to land and wait for 2 days to perform some simple checks, check their reaction...

I don't think "new plane" necessarily means "newly built". A thirty year old Cessna 152 might be new to him.

And "new pilot" is much more of a concern than "new plane".


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Can student hours transfer from Argentina to USA to take the PPL test?
Can the 40+ hours done with a certified flight instructor in Argentina count as the required hours for the private pilots license here in the United States?
The timing of the final test isn't working out and needs to be done in the state if possible. If so, are there any specific documents other than the proof of hours flown and the certification proof of the flight instructor required?  From what I've read it seems like this can be done.  Just need to redo the medical and perhaps the classroom work.  Thank you in advance for your help.

You can count the flight training, per 14 CFR § 61.41(a)(2):

A person may credit flight training toward the requirements of a pilot certificate or rating issued under this part, if that person received the training from [...] A flight instructor who is authorized to give such training by the licensing authority of a foreign contracting State to the Convention on International Civil Aviation, and the flight training is given outside the United States.

Make sure that you are logging your time in accordance with FAA rules, and that your training meets all FAA requirements. Not every country has exactly the same requirements so you may still have things to do.
You'll need to pass the other requirements in the US. Namely, you'll need to get an FAA medical, pass the written test, and receive the required endorsements to take the practical test.

Thank you so much.  It seems the logbook will be in compliance with the info provided.  Do you know if it will be an issue that it is Spanish?

@MarciePyke I don't know of any regulatory requirement that it be in English. But of course your DPE needs to be able to read it. It would be a good idea to make a new logbook with the entries translated to English. Though you'll still need to bring your old logbook that has your CFI's signatures in them.

Thank you again!

I don't know about records except it has to be acceptable to the administrator (aka DPE).  I do know that the practical test including the oral must be done in English.  There is no exceptions to this.  If the examiner believes the applicant is not English proficient, there are guidelines that will allow them to make a more substantial evaluation.


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How I determine numerically the pitch rate of an aircraft?
I have done aerodynamic analysis on a UAV. Is it possible to calculate pitch rate and yaw rate of my UAV depending on design parameters? (I have aerodynamic analysis of the wing, CL, CD, MAC, W, etc.)

What do you mean with "aerodynamic analysis"? Experimental or analytical?

steady state CFD

Well, then you just have to run two (or more, according to your resources) simulations with say two different aspect ratios and then compare the two results (in terms of pitch/roll rate)

You need to do a static maneuverability analysis.  This involves knowing the size of the control surfaces, calculating their effectiveness (which is not easy), some knowledge of the moments of inertia of the aircraft, etc.
In general, this is covered in textbooks on Stability and Control, alternatively, flight dynamics, or flight mechanics.  Roskam has a two-volume set, Etkin & Reid is another -- there are many to choose from.


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What is the unit of the DAP MCP/FCU Selected Altitude as transmitted by the transponder?
The ICAO document Mode S Downlink Aircraft Parameters Implementation and Operations Guidance Document mentions the DAP Selected Altitude.
Is the unit of this DAP always in flight level (i.e. without QNH correction), in feet (i.e. including QNH correction) or is it whatever number the pilot entered in the MCP or FCU? If the latter, how can I (or an automated system processing the DAP) tell if the value transmitted by the transponder is with or without QNH correction?
The use of altitude seems to imply that it's always the QNH corrected value, but I cannot find a definitive reference that clearly indicates the unit of the transmitted value, neither in ICAO nor in Eurocontrol documents.

It's the altitude in feet or meters, depending on the display setting. It is the value displayed in the MCP - either entered by the pilot or provided from the FMS depending on which is the active nav source.
Whether it's QNH or QNE or QFE depends on the airspace the aircraft is (or will be) in when it reaches the set altitude. The autopilot will guide the aircraft so that the altimeter matches the setting. Which altitude that is is determined by the setting of the altimeter reference ("STD" for QNE or the local altimeter setting for QNH or the setting for QFE.)

That it's either feet or meters is really surprising to me. The mentioned ICAO document states in A3.2 (table 2.2) that the selected altitude in BDS 4,0 is 12 bit with an LSB of 16 ft.

Otherwise, in the end it seems that the selected altitude must always be interpreted together with the barometric pressure setting (BPS) in the same BDS register. So to get the value as a "real" altitude, the value needs to be reverse QNH corrected with the BPS given in the same register.

The pilot can select to display altitude in meters when flying in locations where that is the standard. That doesn't prevent the system from converting it to feet in the Mode-S downlink. Depending on the system, it's entirely possible that the system is using feet internally all the time, just converting the display output to meters when selected.

You are correct that to be accurate, you need to account for the BPS. But typically ATC wouldn't care. Their only concern would be that it matches the altitude assignment. They would expect the pilot has the correct BPS.


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Has the inceptor (control stick) ever been controlled via using CAN bus control?
The CAN bus is being used widely in the automotive industry, and some aircraft are using the CAN bus control.
Is there any inceptor (control stick) or throttle that can be connected to the CAN?

Might be worth clarifying - is this trying to find one for use in CAN based simulator project (so does not actually need to be from an aircraft) or looking for in use aircraft that CAN bus the controls?

@GremlinWranger Thanks for the comment. Yes! It is actually for the usage of simulator project. I'm looking for both inceptor and the throttle which are running based on CAN bus.

not my domain but suggest looking for industrial equipment controllers rather than aviation for your sim project. They will be cheaper than aviation and more robust than consumer USB devices.


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Requirements for logging practice instrument approaches (and holdings and course interceptions) while in VMC
Does anyone know the FAA rational for requiring the second pilot on board to have access to a complete Fully functioning set of flight controls in order to act as safety observer for the purpose of logging instrument activities with a view limiting device in VMC?
I have a Long EZ, which has a flight control side stick in the rear cockpit, but no rudders or throttle. I have been told I cannot log practice approaches in VMC with a rated pilot in the back seat because it does not have a complete set of flight controls. This is (as far as I can see from FARs), documented in 14 CFR § 91.109, (c), which reads:

To address comments below, please see
FAA interpretation of "fully functioning dual controls" and Final FAA Interpretaton.
The first FAA Letter above basically states that brakes are not required, but that pitch, roll, and yaw controls are required. It does not discuss or mention power.
The second reference is the final FAA order, (Bulletin HBGA 00-08) it does specifically discuss the throttles, and then states:
Office of General Counsel clarified its position that
the term “dual controls” as used under 14 CFR section 91.109(a)
refers solely to the flight controls of an aircraft (e.g., pitch,
yaw, and roll controls)

As it would seem that the purpose of the safety observer is observe the sky for other traffic and, in the event of an impending or threatening conflict, either

Inform the pilot to take off his/her view limiting device and deal with it, or ...
Take the controls and maneuver the aircraft to avoid the conflict.

I do not understand how rudders or throttle in any way impede the capability of the safety observer to perform that function. It's also curiously inconsistent with the exception made for aircraft with throwover control wheels, (like in Bonanzas).
Also, because I fly in Arizona where the flight conditions are (thankfully), IMC only 25-30 days a year, and because of the wide diversity of avionics in so many GA aircraft nowadays, because of this constraint, my only alternative is to perform practice approaches in another aircraft with different avionics and autopilot switchology from what is in my primary aircraft. With modern avionics, basic instrument procedures and instrumentation now represent an ever-increasingly smaller percentage of the knowledge and skills necessary for IFR flight, so satisfying my IFR currency requirements in an aircraft with very different avionics becomes more and more counter-productive instead of actually increasing my proficiency.
Does anyone know the actual FAA rational for this constraint?

The part about throw-over controls was clearly intended to allow certain Beech planes as an exception and should not be considered indicative of the overall intent.

@Stephen, Try that argument in any court ... "Well, judge, this law was clearly intended ... "

The sentence you reference in paragraph (c)(3) is about a "throwover control wheel". What aircraft do you know of has one of those that controls the rudder? The first sentence references fully functioning dual controls, and that phrase is defined in the FAA bulletin, (granted, aimed at another issue), to explicitly require pitch, roll and yaw but not brakes.

@757toga, I freely grant that this is totally not definitive (as FARs in general), but that's exactly why I'm trying to nail down the reason. But simply "interpreting" the semantics so as to mean what you think makes sense, (as aside, I agree with you - that's why I'm pursuing this!), does not help. The endgame here is to get them to reword the FARs so they clearly and unambiguously define what is required in each of the scenarios we are discussing, not to just give me personal justification to do what I think is right in spite of the FARs.

Hadn't noticed that to be honest!  The language there, in context, supports your interpretation as to their intent, but, again, it is not at all explicit that their intent in (c)(3) is to not require access to rudder control for safety observer during simulated instrument flight.

Really it's just more outdated bureaucracy, a minimally viable rule made worse with time. Don't get me wrong, in most cases I'd settle for the outdated trash over most regulatory dumpster fires started as a replacement. I'm just stating the "reasoning" may be lost to the ages or may have been nothing more than adding a rule because somebody really likes rules(aka HOA board members).

You won’t get an authoritative answer on this unless the FAA publishes their rationale, and someone would have cited such by now if it existed.
However, we can deduce a few things from what is and is not required to act as a safety pilot.  In particular, they do not need to have any special endorsements (such as complex or tail wheel) nor landing currency, nor do they need to have brakes available, but they do need flight controls and a clear view.
From this, I think it is fairly obvious the intent is that the safety pilot can quickly take over the flight controls in an emergency, such as to prevent a mid-air collision or a crash if the primary pilot gets disoriented, but then give them back to the original pilot as soon as the emergency is over, i.e. they’re not expected to land the plane.
To deal with such emergencies, which may necessitate a rapid climb or recovery from a stall or spin, they will need access to the rudders and throttle.
The case of a throwover yoke is clearly not ideal here, but apparently someone decided it can be done quick enough for the safety pilot to do this duty.

I absolutely agree with your logic. But cannot find a definitive FAR or FAA Circular or other document to support this. I asked only because I have been told by some CFIs that "fully functioning" requires rudder control and a throttle... Wish I could nail it down. Thanks for your answer!

Because, for all intents and purposes, there must be a appropriately certificated and rated PIC with the ability to fully control the aircraft at all times during operation.  If you’re flying under the hood but don’t yet hold an instrument rating, you don’t yet qualify for that.  I’ve never had anyone explain a full set of rationale for that regulation. But that being said it’s the offer full control of that aircraft at all times in case something happens with the guy under the hood.  Spatial disorientation at low altitudes and/or high alpha regimes for flight would demand a recovery that requires immediate access to all flight controls and throttle; there may not be enough time available to tell the guy upfront to take the hood off and recover.

Considering that I am asking "why" the FAA seems to have this rule, that might very well be the answer.  Do you have a reference for this? I want to know why before I start asking them to modify it. Clearly rudders are no more necessary than brakes to recover quickly from an upset situation.

If the safety pilot needs to initiate a go-around immediately, he would have to have access to the throttle. Go-around pitch with on-glideslope power would lead to a stall pretty quickly, and "I have the aircraft" works much better than "I have the aircraft, set go-around power for me" when time is short.

@Carlo,  I suppose it might be that recovering from spatial disorientation or performing a Go-around due to a poorly executed instrument approach might be the justification for requiring a safety pilot be in the aircraft when in simulated instrument flight in VMC, but my understanding is that it is because of the potential for mid-air collisions when under the hood in VMC. I mean, there is no safety pilot required when flying instrument approaches in IMC!


text:
Why are there no Mach 4+ fighter planes?
Why are there no Mach 4+ fighter aircraft?  It seems that such aircraft would have massive advantages when it comes to being able to fire longer range missiles and evade return fire.

To fire a longer range missile you need a big missile and big radar. To evade return fire Mach 2+ would actually make it harder.

I think the only Mach 3+ fighter is the MIG-25 Foxbat... There was a YF-12 based on the SR-71, but it never saw production.

See https://aviation.stackexchange.com/questions/21987/why-is-the-maximum-speed-of-current-fighter-aircraft-lower-than-those-from-the-1 I would also add, another disadvantage is the complex and heavy variable geometry intake required to fly that fast has to be carried for the entire mission, but is hardly ever used, as flying at max speed was so infrequent.

Today's fighter are designed with a max speed around mach 2 (max speed is generally calculated with no emport and 50% fuel, the operational top speed is way lower). They keep this ability to pass Mach 1 only to be able to respond fast enough to intercept / air policing missions.
Countries do not design very fast fighters (and planes in general) because :

It's extremely hard to achieve.
It does not give any advantage.
It has no use anymore.

Back in the cold war period, radars and SAMs where not very sophisticated yet, they did not see very far and Ground to Air missiles were not able to get very high. So to protect their airspace from potential intruders / enemy bombers, countries were relying on interceptor aircraft, a fast jet able to catch up the target before it gets to its objective.
At this time, going very fast, very high was an advantage : SAMs where not threatening. The US made a very successful reconnaissance plane exploiting that: the SR-71 Blackbird (max speed : mach 3.3). Many missiles were fired towards the SR-71, none reached its target.
Speed was used for intercept and reconnaissance / intel.
But radars and SAMs improved, getting more range, becoming able to threaten very fast planes. Also satellite images got better and better. The need for very fast interceptors decreased as radar range increased, and the need for very fast recon aircraft decreased as satellite images got better.
Now why does going very very fast not give any advantage to a fighter? Because you cannot turn.
The thing to keep in mind is G force: force the same turn rate, the faster you go, the more g you have to take. And a human can sustain a very limited amount of g. This means a plane going at Mach 4 will barely be able to turn without blacking out its pilot. And if you can't turn, you won't evade enemy missiles, making useless the bit more range gained on the missiles.
And then there is the technical challenge, dealing with a little drag as possible (that means no missile / bomb / fuel under the wings), heat, airframe deformation and wear ... for an advantage which isn't there anymore.
Despite all that, the US are suspected to work on a Mach 6+ aircraft, probably unpiloted.
(Please keep in mind that I simplified a lot of things to not make this post too complex)

@Koyovis thank you very much for the edit, will pay more attention to the his / its thing.

It's a little questionable if a interceptor is a fighter.

@user3528438 Generally a difference is made (or was made before the multirole fighter area) between interceptors and air superiority fighters, one carries some missiles and goes fast, the other carries a lot of missiles and is very manœuvrable.

What about a remotely-piloted drone?  Those don’t have the same G-force limitations as piloted aircraft.

@Demi Indeed, with a drone you lift the human G limitation (which is between 7 and 9), but the structural G limitation is immediately after. Today, most fighters are limited to 9G to preserve aircraft's structure, with option to go to 11 or 13 in case of emergency (which will make full aircraft inspection mandatory after landing to make sure nothing got damaged).
TLDR : you can pull some more g with a drone, but not a lot.

The planned successor to the SR-71 was supposed to reach Mach 4 to 5 but was never completed because of projected costs equivalent to almost 20 billion in today's dollars.
But that was a reconnaissance platform. A fighter has to do more than fly fast and shoot pretty pictures.
Agility would come first on this list: The realistically possible turn rate of a Mach 5 airplane would let it complete a full circle in about 800 seconds, that is almost a quarter of an hour, just for a single circle. If you want to turn any quicker, you better slow down, complete your turn, and speed up again.
(NASA) A Mach 4+ turn needs a lot ground, the SR-71 routinely crossed multiple smaller countries to execute a 180.
Visual contact is another requirement in todays muddled conflicts. In order to avoid collateral damage (and embarrassing political situations), it is often demanded that pilots visually identify a target before opening fire. Try that at Mach 4! Stand-off weapons will be of no use in this situation, and top speed will be irrelevant.
Small size would maybe come as a surprise, but the faster aircraft tend to be bigger and heavier, too. This will make them much more expensive and fewer will be procured. The specialty materials needed for the extreme flight envelope will make them maintenance-intensive, too. This will make them high-value assets that cannot be risked in combat. In the end, the fighting will be done by the smaller, more numerous and less expensive platforms. Why then fund such an expensive diva in the first place?
Operating altitude: If you look at the envelope of the SR-71, you will notice that flight speeds above Mach 3 required it to climb above 65,000 ft. Only then is air density low enough to lower drag sufficiently for prolonged Mach 3 flight. A Mach 4 design would need to climb to 90,000 ft to play out its design speed. What altitude will the adversary be at?

In addition to the other answers: at high speed, the aircraft is heated by air compression. At Mach 2.2, skin temperatures become so high you can't safely use aluminium anymore, you have to switch to steel or titanium, both of which are expensive to manufacture. The SR-71 was built in titanium. It also leaked like a sieve while on the ground, heat expansion at speed sealed those leaks. The SR-71 had to use a special fuel to make that an acceptable risk.
At Mach 4 this issue gets worse again, and you may have to use active cooling or fragile heat shielding to keep your airframe from melting.

Probably the best answer to this question is that, up to this point, there really has not been a need to build fighters capable of those speeds to counter the air forces of friendly nations.
Prior to the mid to late 1970s, the USAF was quietly making preparations at the behest of several aircraft OEMs to make the transition from manned aircraft into high speed missiles for both strategic and tactical uses, given the tremendous punishments both US and modern western air power endured against Soviet radar guided missiles in conflicts like Vietnam and Yom Kippur.  The costs associated with developing a high speed manned aircraft which could evade these threats was prohibitive when compared with the costs of developing a disposable missile to counter it or any future threat, hence the reasons the XB-70, among other programs, were cancelled.  However with the advent of stealth technology in the late 1970s and 1980s, the radar threat was effectively nullified and the exact details of how to build effective stealth aircraft remained out of adversarial hands for the past 20 years, giving us a tactical advantage over existing platforms and making the expenditure of national treasure on a high speed manned platform a moot point.
But that's not to say high speed aircraft are dead.  The Lockheed Skunk Works is currently developing a new unmanned reconnaissance aircraft, dubbed SR-72, capable of Mach 6+, using a combined cycle engine.  Hypersonic aircraft have been proposed over the years for troop or ordnance delivery and there is good evidence that the US and China are currently developing hypersonic air-to-air, surface to air, and ballistic missiles.
It may be that the quantum leap that stealth technology gave us is what delayed the development of high supersonic and hypersonic manned combat aircraft.  And, as potential threat nations develop and perfect their own stealth technology, nullifying the advantages it gave us, that we begin to search of other options like additional speed to retain the tactical advantage.

True, except that a Mach 6+ stealth aircraft isn’t possible, as it will be a massive IR emitter.

Not necessarily

Fuel consumption and combat range. Going to mach 4 it would be a problem for the fuel consumption.
Some legends, they say something there is but we do not know it :-)

Are you aware that there is such a thing as air to air refueling?

@KorvinStarmast: Yes.  Are you aware that air-to-air refueling requires air tankers (AKA big fat targets) to be staged in appropriate spots, or to accompany fighters, thus limiting the squadron's speed to that of its slowest member?

@jamesqf I am aware of how tankers actually are used IRL.  Have done so.  You?  Where you position a tanker is an art.  There is no need to run your Tankers into red air space ... On the other hand, this whole discussion (mach 4 fighters) is moot IRL.

@KorvinStarmast: Why is red air space even an issue any more, since there are really no more reds, other than a couple of islands and penninsulas that could be missed if you blink at mach 4?

@jamesqf red being a generic term for foe/enemy. You could use orange as well.


text:
Why do some aircraft carriers have an upward starting runway?
I noticed that some aircraft carriers have an upward starting runway while others don't. What's the reason for this? What are the advantages of both designs and how come not one of them has prevailed over time?

The upward slope on the runway is also known as a "ski jump", and it is to give VSTOL (Vertical or Short Take Off and Landing) aircraft get aloft on the short carrier deck. Carriers without this use aircraft catapults to quickly get airplanes up to flying speed.
The advantages of the ski-jump are its simplicity, which means the carrier has lower build and operating costs. The drawbacks are that VSTOL airplanes are generally more limited in their capabilities, the airplanes that are launched by catapults pack a lot more punch.

Thanks! Do you also know why only the US army is not using ski jump runways? Apparently every other nation using aircraft carriers uses both.

The US Navy prefers to spend more on carriers so that their airplanes have more capability, French carriers also do not use ski-jumps

It is more like the US Navy can afford to build aircraft carriers with more than twice the size of any other carrier. Their "landing ships" are bigger than the carriers of other nations. Everything's bigger in the US...

There are two basic designs to large deck aircraft carriers operating conventional fixed wing aircraft these days. The first is the short take off but arrested landing (STOBAR)  and the catapult assisted take off but arrested landing (CATOBAR) designs.
The STOBAR designs, such as the Russian Navy's Admiral Kuznetsov utilize a ski jump or an upward sloping flight deck at the bow of the ship to launch airplanes via a conventional short take off.   The ski jump design offers an increase in vertical velocity to the aircraft just prior to take off giving it more time in the air as it goes off the bow to build up more endspeed to fly.  Jets start their takeoff rolls either at launch stations at the bow or the ship's waist equipped with jet blast deflectors (JBD)  and hydraulic retractable chocks  on the main landing gear to act as holdbacks so the plane can go to full power prior to the takeoff roll.
CATOBAR designs don't utilize a ski jump for an assist,  but instead make use of an aircraft catapult either in the form of a steam powered design and also in the form of a large electromagnetic linear motor on newer ships call electromagnetic aircraft launch systems (EMALS).   Both systems essentially boost the jet to its required endspeed  over launch track of about 100 or so yards.   Modern CATOBAR designs like the US Navy's Nimitz or Ford-class carriers have four such catapults, 2 on the ships bow and 2 on the waist, allowing for up to four aircraft to launch at once.  Each cat is equipped with a JBD and makes use of a frangible holdback attached to the nose landing gear.  The catapult connects to the jet via an above deck shuttle which attaches to the jet either by means of a wire rope bridle or a steel launch bar on the plane's nose wheel.
Both CATOBAR and STOBAR designs recover aircraft in the same manner. The ship is designed with an angled landing area and makes use of hydraulic arresting gear in which an airplane will catch using a steel tailhook to stop in a short space.
STOBAR designs offer a much simpler system with less moving parts requiring maintenance than CATOBAR designs.  Conversely CATOBAR designs  can launch aircraft with much higher useful loads (fuel and weapons)  as well as offering the capability to launch over twice as many aircraft at once as compared with the STOBAR system.


text:
What are the mandatory elements of an ICAO flightplan?
I need to write a software parser for ICAO flightplans, and in particular I need to parse FPL messages.
What are the mandatory elements included in a flightplan document? For example the image below illustrates a typical flightplan form:

Also the above form contains special sequences such as <=. What is the signification of this sequence? Does it correspond to a newline in the generated flightplan text format?
A corresponding textual representation of a flightplan is also shown below:
(FPL-REU974-IS
-B77L/H-SDE1E2E3GFHIJ4J5M1RWXYZ/LB1D1
-LFPG1845
-N0490F310 OKASI UL612 MILPA UM730 TOP UL50 ELB UL12 VELAD UM728 NERAR UP3
RCA/N0489F350 UR611 TIKAT UG300 MAV UM665G ITLOX UM665 UVESO/N0486F370 DCT DENLI
UR780 MIDRI UR780G UVENA
-FMEE1036 FIMP
-PBN/A1D1L1S1 NAV/RNP10 DOF/121114 REG/FOLRA EET/LSAS0039 LFFF0039 LIMM0048 LIRR0111
LMMM0218 HLLL0237 HECC0343 HSSS0425 HAAA0545 HKNA0700 HCSM0701 FSSS0745 FMMM0900
SEL/CGFR ORGN/RUKOUU PER/C SRC/RQP RMK/ADSB ACARS EQUIPPED TCAS EQUIPPED)

So basically I need to write a program capable of parsing the previous textual format and validating it against any errors and or missing elements.

Thanks for the links. The second link mentions mandatory and optional fields in the ICAO flightplan form. However an example of the form is not available, which leaves me still wondering what is mandatory?

Thanks for all the info. I suggest you put your comments in an answer so than I can accept it

replaced FAA template with ICAO using link to Doc 4444

About the special characters at the beginning or in the body of the form: They are:

Formatting characters inserted when sending the FPL with a teletype interface on the AFTN network. e.g. < is a carriage return, = is a line feed

Routing indication, e.g. FF is the priority level for a message of type FPL.

These characters are not part of the flight plan data of course.

Parser assumptions

All fields from 7 to 18 are mandatory and included in a Filled Flight Plan message (FPL). Being mandatory doesn't prevent some field to be filled with some dummy value, e.g: Field 18 may contain "0" to indicate no information is provided.

Field 19 can have survival items and remarks left blank. Field 19 is not part of the FPL message, but of the SPL message. The latter is transmitted to other ATS units by the ATS unit having registered the flight plan only in specific situations, e.g. when the contact has been lost with the aircraft (ALERFA).

ATS consistency
While they are mandatory, some fields are used only in specific conditions, in particular the ones related to navigation capabilities.

Example: Not all approaches or routes are PBN required. Not mentioning some navigation capabilities may not be a problem if the capability is not required by an ATS unit to provide the desired routing, but if you use a RNAV approach, the flight plan will be rejected.

For more information on the combinations that are consistent, see the interactive Eurocontrol Flight Plan Guide for IFPS (Integrated Initial Flight Plan Processing System). Example:

When PBN is specified in the item 18 of a flight plan, it indicates the Performance Based Navigation (PBN) levels and/or the Required
Navigation Performance (RNP) levels that can be met. It can only be
present if the aircraft is PBN approved which shall be notified by the
letter R in the equipment of that flight plan.

**Filling instructions**
The content of the ICAO flight plan is detailed in appendix 2 of ICAO Doc 4444 (PANS-ATM) and a filled sample is provided (page 361), but filling instructions are also available separately, e.g.:

ICAO Flightplan Form Basics
Flight Plan Filling at SKYbrary (Eurocontrol)

Note that FAA and ICAO flight plan forms have been different in the past, and while FAA is pushing for the adoption of the ICAO form, it seems the FAA form is still used for US domestic flights.


text:
Where can I find a reliable database of civilian ATS Routes?
I am looking for an updated source of ATS routes. Are there any free and reliable (ie produced by the FAA or Eurocontrol) datasets available?

Ideally I am looking for a dataset that I can readily import in a database (for example a csv dataset).

Does not exist.

OK. So how do airlines proceed? Do they build there own ATS Route database?

@BigONotation The data is licensed or sub-licensed from a provider such as Jeppesen, LH Systems Lido, or NAVBLUE.

Thanks. Please add your comment as an answer so I can accept it


text:
What is the difference between pilot recency, recurrent checks, qualifications and ratings?
I have heard the terms pilot "recency", "recurrent checks", and "qualifications" but they seem kinda fuzzy to me. How do these terms relate to each other?

I think to be clear, a lot of terms are used relatively casually and the strict legal terms might differ. But, to try and give a layman overview:

Recency (Otherwise known as Currency)

The idea is that skills fade over time - as such, most pilots will need to fly a certain number of hours in a certain timeframe. For example, in order to take passengers in the UK you MUST have conducted 3 landings and take offs in the last 90 days. If you haven't, you're "out of currency" and can't take passengers flying until you do. Beyond that, insurers, employers, flying clubs etc might impose stricter requirements. For example - they might request you take a flight check with an instructor if you don't fly 1 hour each month.
These requirements are based on your hours logged - they aren't usually formalised in terms of needing to fill in a form. You just need to, when asked, be able to demonstrate your "currency"

Recurrent Checks / Training / Reviews

This would describe a flight with an instructor or examiner (Depending on who can approve what) to maintain an existing rating or privilege. You aren't getting anything NEW for doing it, but it's a formalised check of your ability. You can think of them as mini flight tests if you like - you have to book it, it'll be signed off and they're usually legal requirements. Though, again, companies may have their own policies too on top of that.

Qualifications

This is very broad, and I suspect means different things to different people. I would say a qualification is any privilege that you specifically hold - whether that be your licence, your rating, endorsements or anything else.

Licences

Broadly, your pilots licence is split into two parts. You have a "Licence" of some description, and this describes what kind of pilot you could be. For example "Private", "Commercial", and "Airline Transport".
They do not describe the TYPES of planes you can fly - just that if you did so, you could exercise the privileges of either a Private Pilot, a Commercial Pilot or an Airline Transport Pilot.
Here in the UK, your licence is granted for life - once you have a PPL you are a private pilot forever. But that doesn't mean you can fly, because the second half is Ratings, and they can expire (as above, with lack of currency):

Ratings

Ratings describe aircraft you are trained on. Most jurisdictions start with a simple "Single Engine Piston" rating which covers you to fly any light aircraft, with a single piston engine. This is what you receive when you get your first pilots licence and it's this "rating" which actually lets you fly. Beyond that, ratings tend to fall into two further groups:

Type Ratings - To cover specific aircraft. I.e., you would need to train and receive a "type rating" on a Boeing 747 in order to fly a 747.
Class Ratings - To cover a specific configuration of aircraft. For example - instrument, multi engine, multi crew, single engine turbine. I.e., you would also need a "multi crew" and "multi engine" class rating, to fly your 747 on top of your type rating

Endorsements

Endorsements are a bit of an "other" category and are handled differently in different jurisdictions. You can think of them as being "signed off" to do something - but there's usually not a formal examination. This is generally only seen in private flying and will include things like high performance aircraft, complex aircraft (retractable undercarriage, adjustable prop) and taildraggers

Thanks a lot! Your answer clarifies the terminology a lot!


text:
What materials are used for the gas bags in hydrogen airships?
I heard that to construct a hydrogen blimp or airship, the gas bag must be coated with aluminium, which got me confused: doesn't it add significantly to the weight? Is aluminium really used, and is there a specific reason to use it?
I'm also interested in knowing what is the best lightweight material in general for building an airship/blimp bags.

Why would you think that any coating applied to the structure wouldn't add weight? What do you mean by payload? Its lifting capacity?

@RonBeyer Yes i also thought too myself, it'd add weight to the airship. Can you please provide relevant links or example to bag materials

Its not really called a "bag", its a cell. A single airship is compromised of a number of cells to provide the lifting capacity. I'm still not entirely sure what you are after though, everything you add to the airship beyond the lifting gas adds weight, the trick is having more lift than weight (or equal amounts). I'm not sure there is a "best" material as it may depend on the application...

@RonBeyer Exactly, considering my question, what material should i use to hold the hydrogen in the "cells", any relevant links ?

Aluminum coating actually means some microns of aluminum (vapor deposit). Aluminized PET is known for its impermeability to gases.

@mins thanks for the note, if am right by what your saying, aluminium is used to tighten the gas inside the bags so that it doesn't diffuse or escape

See this article, e.g. table 1. This is the "permeation" rate from one side to the other for different substances. It just means it's more airtight, and in addition its reflective properties limit the temperature (hence pressure) inside which may also help to prevent permeation. Metalized PET or mylar (BoPET) are commonly used for that.

Have you ever heard of goldbeater's skin? This was the preferred material before synthetic materials became available.
What is it? Wikipedia says:

Goldbeater's skin—the outer membrane of a calf's intestine—is a
  parchment traditionally used in the process of making gold leaf by
  beating, reducing gold into mere 1 μm-thick leaves. […] To manufacture goldbeater's skin, the gut of oxen (or other cattle) is soaked in a dilute solution of potassium hydroxide, washed, stretched, beaten flat and thin, and treated chemically to prevent putrefaction.

From the 1880's on it became the preferred material for gas bags, displacing rubberized cloth or silk which would become brittle quickly. The first man-carrying hydrogen ballon used already silk which had been rubberized by varnishing it with rubber that had been dissolved in turpentine. Again Wikipedia:

Large quantities of goldbeater's skin were used to make the gas bags
  of early balloons created by the Royal Engineers at Chatham, Kent
  starting in 1881–82 culminating in 1883 with "The Heron", of 10,000 cu
  ft capacity. The method of preparing and making gas-tight joins in the
  skins was known only to a family from Alsatia called Weinling who were
  employed by the RE for many years. The British had a monopoly on the
  technique until around 1912 when the Germans adopted the material for
  the internal gas bags of the "Zeppelin" rigid airships, exhausting the
  available supply: about 200,000 sheets were used for a typical World
  War I Zeppelin, while the USS Shenandoah needed 750,000 sheets. The
  sheets were joined together and folded into impermeable layers.

There is a more detailed description in the Wikipedia article on the ZR-1 which reveals that the goldbeater's skin was reinforced with cotton cloth:

The gas cells were made of goldbeater's skins, one of the most
  gas-impervious materials known at the time. Named for their use in
  beating and separating gold leaf, goldbeater's skins were made from
  the outer membrane of the large intestines of cattle. The membranes
  were washed and scraped to remove fat and dirt, and then placed in a
  solution of water and glycerine in preparation for application to the
  rubberized cotton fabric providing the strength of the gas cells.
  The membranes were wrung out by hand to remove the water-glycerine
  storage solution and then rubber-cemented to the cotton fabric and
  finally given a light coat of varnish.

Today the preferred material is polyethylene therephtalate, a polyester, which is also used for blister packages or soda bottles. By itself, it would have far too high a rate of permeation, therefore it is sealed with a metallic layer of a thickness of only a few atoms. Such metallized films are also used for thermal blankets, in spacesuits, on satellites and for toy helium balloons. To increase its tensile strength, the film is stretched in two perpendicular directions such that the molecule chains become elongated and the film becomes thinner. The result is called biaxially oriented PET (BoPET) and metallized by vapor deposition of a thin layer of aluminium.

"The outside of the envelope is coated with aluminized paint for protection against sunlight." (emphasis added)
Read more: http://www.madehow.com/Volume-3/Airship.html#ixzz4j8ekSPPL

I'm not sure how your volume calculation applies to the question.

Well, the aluminized paint was less for protection than for preventing heating in sunlight, like it is done with aluminized plastic foil on spacecraft. Especially when flying in partly cloudy conditions, sun heating would make altitude control impossible.


text:
Are some airline's flights more noisy than others for security reasons?
LHR has published a table ranking the noise and pollution levels of the different airlines using the airport. At the bottom of the list is Israel's El Al.
El Al, in their official statement, say that this is because they are using older 747-400 planes which are due to be replaced by Dreamliners. However, somebody was trying to explain to me that the extra noise was due to particular flight methods and manoeuvres which they implement for purposes of security and defence, being a high risk airline.
I'm not sure what they could do differently, especially as they should be under UK ATC control.
Does anyone have any concrete knowledge (obviously not classified info) or an educated guess whether this guy's theory is rational or not.

Sounds like nonsense to me

Quite happy for it to be nonsense or urban myth. It has more weight if professionals say so than if I say it myself.

The only credibility I can give is if the airline does a quick climb-out or a steep descent to reduce surface-to-air vulnerabilities, however even hand-held SAMs can shoot down aircraft at cruising altitude. I guess they could also be trying to avoid small arms fire or RPGs.

I don't know if this specific case in an urban myth but it is not unheard of that some airlines on some airports have used non standard departures in order to be less predictable and therefore are at lower risk to be exposed to MANPADs. Of course ATC is involved in such schemes.

I have read that aircraft in Afghanistan use maximum climb rate to get out of range of small arms fire, but I can't imagine that would be a concern at LHR .

There are a limited number of "standard departures" from Heathrow and the corridors are incredibly tight with 8 busy fields nearby who routes need to co-ordinate with Heathrows. No way would dispensation be given to a particular airline, regardless of being "high risk".  And all this talk about manpads, small arms fire and RPGs. We're talking London, not Baghdad.  You have no chance of bringing down an airliner in flight with an RPG or small arms and deviating a bit isn't going to help with manpads and guided weapons.

Noise abatement procedures?

In quasi war areas, small arms fire is a constant nuisance.  There are published open literature studies. I am unaware that making more noise helps security, although as others have pointed out, it may be a byproduct of performance climbs to altitudes where small arms hits are reduced.

Having read a ton on ElAl's security measures, of which they have many, I don't recall anything along the lines of this. Most of their security is on the ground against the possibility of a bombing or hijacking of the plane (those people that ask you questions before check-in are amazing at what they do, and read every time your eyebrow moves to see if you're suspicious). They are usually not afraid of SAMs and the like, and if they are, they wouldn't fly to the airport where there is a threat of it.

Being familiar with neither El Al nor LHR, what follows is entirely in the category of "educated guesses".
There are two broad ranges where noise levels can be affected by operations, those being on approach and on departure.  The things that tend to make aircraft noisy on approach would be configuring early and flying down final with lots of drag (i.e. full flaps) and lots of power (i.e. lots of engine noise).  On departure, using maximum thrust instead of a reduced thrust takeoff tends to be more noise close-in to the airport, but it results in a steeper climbout, which makes for less noise farther away.
Would "standard operating procedures" for a "high risk" airline tend to require either of these?  Hard to say.  As far as survivability, configuring early means that if you take a missile close in, you're already in a landing configuration, so losing a hydraulic system or two doesn't affect your ability to get configured for landing.  On the other hand, being at full flaps makes performance on a go-around worse if you take the missile and can't change configuration (but need to go around rather than land).  Plus, engines running at the higher power setting would tend to be more of a target for an IR (heat-seeking) missile.  If you thought you were coming in to a high-threat airfield, you'd probably want a steep approach, meaning lots of drag but a LOW power setting.  I have a hard time envisioning a 747 doing this coming into LHR, but maybe they do, spooling the power WAY up to arrest the steep descent at some point close-in.  Maybe.
On departure, the max-thrust takeoff DOES get you out of some threat envelopes more quickly than the reduced thrust takeoff, so that's fairly believable.  Get as high as you can before you depart the airport perimeter (which is presumably more secure than the areas outside the airport fence).  If "always fly a max thrust takeoff" is part of the El Al S.O.P., then it wouldn't be surprising that this would result in higher noise levels close in.
Still in the "educated guess" category, I would wonder if some element of the noise ratings may have to do with how they're calculated.  If the rating for British Airways averages out the 747's departing at heavy weights to destinations far away (four engines at very high power settings putting out lots of noise) with smaller, lighter airplanes departing to closer destinations (two engines at reduced thrust, putting out less noise), then their average would be lower than a carrier which is always flying heavy 4-engine jets to a fairly distant destination.  Not that El Al would be the only airline in this situation, I'd suspect, but not having smaller & less noisy aircraft to reduce the average may be at least one factor in the ratings.
Without seeing the actual ratings and at least some of the methodology as to how they're calculated (near/far, approach/departure/both, weighted by time of day, whatever else), it's very hard to know what's really going on there.  Plus, taking a look at the procedures into LHR & seeing if there is any difference in how El Al flies them and how "everybody else" does.  A lot of questions there in order to get a true high-quality answer to what's really driving those results.

Something tells me, that for an average airliner, taking a single missile in any configuration would spell disaster. They aren't designed to evade, or confuse any incoming missile (be it IR, RADAR, or other)... and the structure isn't designed with partial breakup survivability in mind. My guess would be, any "security" measures being taken are for reducing damages caused by small arms fire.

@SnakeDoc Stratfor quotes 30% survival rate for civilian aircraft hit by a MANPAD, and notes that El Al aircraft actually do have some sort of countermeasures installed.


text:
Could a knowledgeable passenger have saved Helios Flight 522?
Helios Flight 522 crashed when the crew unknowingly suffered hypoxia. Although the masks in the passenger cabin deployed, the crew were unaware of this and didn't realise the extent of the problem.
If I was a passenger on a plane at cruise altitude and the oxygen masks deployed, I know that there is only about 10 minutes oxygen available. I would expect the plane to immediately descend to a lower altitude.
If I don't notice any change in attitude and there is no passenger announcement after a few minutes, would it help to alert a flight attendant to check that the pilots are aware of the issue? Or would it be best to assume that 'they know what they are doing' and just keep quiet?

Well, the purser was more knowledgeable than a passenger would and he failed to save the plane…

It's never bad to inform a crew member about anything you think may be abnormal.
In this special situation, as you know that the oxygen masks deploy automatically if the cabin altitude raises to high, you expect the aircraft to descend in order to allow the pax to breathe normally again. If this doesn't happen it would be possible that the crew deployed the masks manually for no reason or the automatism faulty detected a low cabin pressure. In both cases I would expect an announcement that the oxygen masks are not needed and can be put down safely.
If neither of those options happens something is definitely wrong. Either the pilots already suffered hypoxia or they didn't notice the passenger masks drop at all.
In my opinion the cabin crew should be trained to know what's normal procedure if masks drop, but I would definitely tell a flight attendant about my concerns if nothing happens after two minutes or so.
You always have to remember: In high altitudes you become unconscious after less than 30 secs without the mask, and as the masks can only produce a limited amount of oxygen it's a really time critical issue to reach the flight-deck and initiate a descend if the pilots are already out of service!

I believe the cabin crew has mobile oxygen tanks with masks which would allow people to move around whilst still be able to breathe.

You don't die! Not that fast, really. You become unusably dizzy and incapable of thinking. You become unable to realize that you should put on the mask. But you really don't die that fast.

Can you actually do anything about it these days though? I thought that since the reinforced doors you can't get into the pilots cabin unless they let you in so if they are both out of commission you're SOL?

What I was trying to say is that they didn't notice that the pax oxygen masks dropped (automatically or faulty) and therefor do not react via an announcement

Edits look good, i have removed my down vote and comments.

Thanks a lot for the tip! Didn't notice it wasn't clear enough.

@DRF I'm really unsure at all, but I think the purser has the code to open the door.

@VladimirF If cabin altitude reaches about 38000 feet you are incapacitated after ~15 secs, and you will die 100% without help of somebody else. But I see your point and edited the sentence

but most people can hold their breathe for more than 30 seconds. how come you loose consciousness so fast?

@njzk2 because in this case people aren't holding their breath. It's like asking why do people drown if they breathe in water even though they can hold their breath for a minute or two.

@njzk2 - you cannot hold your breath. You need to exhale. Pressure outside of your body is so low that your lungs might be damaged [citation needed].

@PeterMasiar you don't really need to have air in your lungs to hold your breath.

@njzk2 You need to, otherwise there will be a reverse process in the lungs and the blood will loose oxygen very quickly. But you will not die that fast at 40000ft. You will die if the pilot is incapacitated and you crash, but not from the lack of oxygen in 30 seconds. Not at that height.

If you are breathing off of a cabin mask, you are breathing at altitude, and you can hold your breath at that pressure absent climbs or descents, but the amount of available oxygen would be reduced and the partial pressure of CO2 would presumably build abnormally quickly, so you could only do so for a reduced amount of time compared to at sea level.

@DRF Cockpit doors have codes to request entry from outside, and the flight attendants know the code. The flight crew can deny entry (which is why the captain couldn't reenter the cockpit on that Germanwings flight), but if they don't respond to the request, the door unlocks.

The mask trigger pressure is a pressure I can breathe at for hours; however 40000 feet not so much. It really does depend on how high the plane is.

@Chris Stratton The CO2 would not increase. That's what makes hypoxia so insidious. Your brain does not register a lack of oxygen in the blood, rather it registers an excess of CO2. That's what gives you the uncomfortable suffocating/drowning feeling when you can't breathe. But if you can still eliminate CO2 you won't know anything is wrong until you start getting confused, etc. I've read that sometimes people find it pleasurable, like being drunk or high.

@TomMcW this concerned holding your breath, in which case there is no place for the CO2 to escape to.  Given it would be less dilute, you might well react to it faster.

@ChrisStratton I see. Yeah, CO2 is why you can't hold breath very long. If it weren't for the CO2 you could probably hold your breath for quite a long time - on the ground, that is.

First and foremost you should read the accident report which you can find in full here. I will pull some excerpts from that below.

Helios Flight 522 crashed when the crew unknowingly suffered hypoxia.
  Although the masks in the passenger cabin deployed, the crew were
  unaware of this and didn't realize the extent of the problem.

This is the one of the debated/interesting aspects of the accident. The crew would have been alerted by alarms to the depressurization event

...Non-identification of the warnings and the reasons for the activation
  of the warnings (Cabin Altitude Warning Horn, Passenger Oxygen Masks
  Deployment indication, Master Caution).

Many might say they did in fact know there was a problem but for some unknown reason they did nothing about it. Generally speaking "Don Oxygen Masks" is the first item on the emergency checklist following the mishap of the South Dakota LearJet crash in 1999. One can conclude from the report that its possible the pilots assumed the cabin pressure system was in the proper place (auto) when in fact it was set to manual (MAN) and subsequently were confused by the alarm or thought it was faulty.

...that the following factors could have contributed to the accident:
  omission of returning the cabin pressurization mode selector to the
  AUTO position after non-scheduled maintenance on the aircraft;

This is a procedural failure on the implementation end. The report would cite bad procedures for this

...the report also identifies a number of additional safety deficiencies
  pertaining to: maintenance procedures; pilot training, normal and
  emergency procedures; organizational issues of the Operator;
  organizational issues related to safety oversight of maintenance and
  flight operations by Cyprus DCA, EASA/JAA and ICAO; issues related to
  the aircraft manufacturer’s documentation for maintenance and flight
  operations;

If I was a passenger on a plane at cruise altitude and the oxygen
  masks deployed, I know that there is only about 10 minutes oxygen
  available. I would expect the plane to immediately descend to a lower
  altitude.

There are some assumptions here of course first and foremost that the plane is still maneuverable. There are any number of things that could cause a depressurization and some may also cause flight control issues as we saw Aloha Airlines Flight 243 a situation where the crew did not immediately know what was going on. However generally speaking, once the issue has been identified the plane should start a decent if possible.

If I don't notice any change in attitude and there is no passenger
  announcement after a few minutes, would it help to alert a flight
  attendant to check that the pilots are aware of the issue? Or would it
  be best to assume that 'they know what they are doing' and just keep
  quiet?

This of course brings up another debate. In almost all cases you should alert the crew to anything you feel is strange our out of place. Of course in this instance it may be hard to move about the cabin, and potentially hard to shout or get your message to the crew. In some cases the crew may be working the issue and getting in the way of that can be problematic. Again generally alerting them is a good thing but its a situation by situation basis for sure.

In regards to the comments on other answers you will not die, at least one flight attendant (who was also a pilot) survived the ordeal and made his way into the cockpit as was heard on the CVR

At 08:54:18 h, the following distress was recorded by the CVR “MAYDAY,
  MAYDAY, MAYDAY, Helios Airways Flight 522 Athens … (unintelligible
  word)”. A few seconds later, another “MAYDAY, MAYDAY” with a very weak
  voice was recorded.When the Boeing 737 was at about 7 000 ft, the
  person in the Captain’s seat for the first time appeared to
  acknowledge the presence of the F-16s and he made a hand motion.

Could a knowledgeable passenger have saved Helios Flight 522?

Unlikely as these days its hard to get in the cockpit, or even get a message to the pilots. But you should, in any event at least try. This saved some lives on Air Florida 90 when a passenger realized icing on the wings and assumed the crash position and informed the other around him.

The pilot did not regain conciousness, it was a flight attendant named Andreas Prodromou with a commercial pilots license (not rated for that aircraft) that was sitting in the pilots seat.

interesting, ill remove that section.

It seems from the Wikipedia page that there was indeed an knowledgeable passenger. Not fully qualified, but ...

At 11:49, flight attendant Andreas Prodromou entered the cockpit and sat down in the captain's seat, having remained conscious by using a portable oxygen supply. Prodromou held a UK Commercial Pilot License, but was not qualified to fly the Boeing 737. Crash investigators concluded that Prodromou's experience was insufficient for him to gain control of the aircraft under the circumstances. Prodromou waved at the F16s very briefly, but almost as soon as he entered the cockpit, the left engine flamed out due to fuel exhaustion and the plane left the holding pattern and started to descend. Ten minutes after the loss of power from the left engine, the right engine also flamed out, and just before 12:04 the aircraft crashed into hills near Grammatiko.

So depending on how knowledgeable you're looking for, this seems pretty clear-cut.
https://en.wikipedia.org/wiki/Helios_Airways_Flight_522

I agree that he was a pilot, but why did he wait until the fuel was exhausted before entering the cockpit? Maybe he didn't know that he should act quickly?

@David Glickman The CVR only recorded the last 30 min of the flight and the F16 intercept did not arrive until 2 hours after the last contact with the pilots. So we don't know what the cabin crew may have done during that period of time. The FDR does not register the operation of the intercom or the cockpit door. It's quite possible the FA's may have tried to call the pilots or entered the cockpit earlier. They may have tried calling ATC for help. The report says that when the FA tried to call mayday it did not go out over VHF, so he was probably not familiar with how to operate the radio.

authors: Vladimir F Героям слава


text:
Why do smaller RC helicopters have visible twist in their rotors, and slightly bigger ones do not?
Smaller, cheap, RC helicopters generally have very pronounced twist in their blades, and also vary their chord - slightly larger, slightly more expensive RC helicopters have no visible twist or chord change in their blades (i also own both varieties, and the larger blades also have no measurable twist - the answer and comments to Why do helicopter rotors have constant section and angle of attack? posit that real helicopters have twist in their blades, so i was unsure whether it is only an optical illusion in RC helis)
The most expensive of all RC helicopters, Ingenuity, also has very visibly pronounced chord and angle changes, so it does not seem to be about price :-)
With the huge difference in airspeed over the inner versus the outer portion (In the example picture, the larger, yellow, helicopters innermost chord should only get less than a quarter of the airspeed that the outermost portion gets) i would expect a more visible twist - is this something that only happens in flight, as a reaction to aerodynamic forces, i.e. does the blade twist while in use? Does changing the chord as well as the aoa pose too big a construction hurdle for the bigger RC blades, while it is easy to do in injection molded plastic for the small ones?

The small helicopter doesn't have a swashplate while the larger one does, this makes their controls function differently

@ratchetfreak - do you mean to say that the controls-scheme is of interest for the twist (chord-change) of the rotor? I mean Ingenuity also has twist and their control scheme is non-standard, so i am not doubting you, but could you go into the why and wherefore?

The difference is due to the different systems of how they produce their upwards lift and is not necessarily linked to the size.
On most small helicopters (like the one you have in the lower picture) you have a so called "fixed-pitch" system. The rotor blades always have the same curvature (pitch) and how much lift they produce is controlled by the RPM of the rotor. This system is easy to produce (and therefore cheap) and it's enough for small (toy-) helicopters.
The "professional class" helicopters (they can be small but also big - usually the big ones are "pro-class") use a different lift-producing system. It's called "collective-pitch". The rotorblades are not angled, instead they are symmetric in their profile and keep 0° pitch (not angled) when the rotor is not turning.
At startup, the rotor spools up and keeps a preset speed but the helicopter itself is not moving. To change the lift, the helicopter starts angeling all of its rotor blades simultaneously and they start pushing the air downwards, lifting the helicopter upwards.
Because the rotorblades have to change their angle (pitch) during rotation, this system is way more complicated to manufacture, but it allows the helicopter to actually fly complex maneuvers instead of just goining up and down and moving on one plane. Collective-pitch helicopters can even roll upside-down and adjust their pitch accordingly to fly loops, etc.

While i see that argument make sense for the case QuadmasterXLII's answer touches as well, i.e flying inverted, i do not really see the relevance for helis that do not flip - collective pitch should enable any sort of blade to have zero lift - on a twisted blade the inner part would still produce lift, while the outer part would already produce negative lift, cancelling each other out. The losses incurred from always having some part of the blade at a suboptimal aoa (as is the case for a straight blade) should outweigh the slight inconvenience of having to overpitch for zero lift, no?

@bukwyrm Having pre-pitched blades on collective pitch doesn't make sense because, first of all, in idle position the curvature would produce a ton of turbulence and with it drag, lowering the efficiency, and second of all, one curvature type is only optimized for a certain pitch. So having curved blades would be good at one pitch setting but suboptimal at all the others

Everyone seems to be saying it has to do with the control scheme and presence or lack of a swash plate. The twist along a rotor blades' length is called washout. The property of getting skinnier as it goes out is called taper. Small RC fixed pitch helis usually have lots of taper and washout, and it makes sense why, since the tip of the blade is going much faster than the root, and needs much less pitch and size to make the same amount of lift. Its a more efficient design if you add taper or washout. Thing is, full scale helis have washout and sometimes taper too, it's just much more subtle on higher speed and larger rotorheads which take advantage of the efficiency of longer skinnier blades. Some micro helis like the scout CX have relatively straight and square blades, and some with a single head, no flybar and a swash plate (such as the blade 120 s2) have lots of taper and washout. I think fixed pitch helis just tend to be smaller and operate at lower head speeds where washout and taper help a lot. Obviously for 3D flying you'd want a symmetrical blade but could still utilize taper.

Historically, there are two main categories of people flying expensive RC helicopters: 3-D pilots who like to do flips and fly upside down, and scale pilots who want to look like real helicopters. The 3-D pilots need symmetrical blades for flying upside down, and the scale pilots don’t care much for gains in efficiency - they just want the same camber and twist as the real helicopter they are modelling, which is subtle. I suspect that the large, straight bladed helicopter you own has the capability to hover upside-down: is this the case?

No, the straight-blade heli is not able to fly inverted, or maybe it's just me :-)

One explanation without diving too deep is sections of bigger rotors will travel at higher speeds so they don't need high angle of attacks while smaller rotors need high AOA to get enough bite

Do you have numbers on that? I have a 20cm-blade-heli with lots of visible twist, and a 30cm heli with none, while, for instance, the props of something like the Osprey are twisted a lot as well, while the Bell-206, with shorter blades, has very flat-seeming ones.